1. Field of the Invention
The present invention relates to a coil component uses as a major component or the like of a common mode choke coil or transformer and a method of manufacturing the same.
2. Description of the Related Art
Reductions in the size of electronic equipments such as personal computers and portable phones have resulted in demand for reductions in the size and thickness (low height) of electronic components such as coils and capacitors that are mounted on circuits in electronic equipments.
However, a wound-wire coil provided by winding a copper wire or the like around a ferrite core has a problem in that it is difficult to make compact because of structural limitations. Under the circumstance, research and development is active on chip type coil components, which can be provided with a small size and a low height. Known chip type coil components include multi-layer type coil components provided by forming a coil conduct or pattern on a surface of a magnetic sheet such as ferrite and stacking such magnetic sheets and thin film type coil components provided by alternately forming insulating films and coil conductors in the form of metal thin films using thin film formation techniques.
As a thin film type coil component, a common mode choke coil is known. The common mode choke coil has such a structure that two coil conductors in the form of spirals provided opposite to each other through an insulating film are embedded in an insulating layer formed between two ferrite substrates (magnetic substrates) disposed to be opposite to each other. Open regions are formed at an inner peripheral side and an outer peripheral side of the spiral coil conductor. A magnetic layer made of a magnetic material obtained by mixing magnetic powder in an insulating material is formed in the open region. When power is applied to the coil conductors, a closed magnetic path is formed on a section including the center axis of the coil conductor.
A conventional method of manufacturing a common mode choke coil will be described with reference to FIGS. 8A to 10B. FIGS. 8A to 10B are manufacturing process sectional views of a common mode choke coil. First, as shown in FIG. 8A, an insulating film 59a is formed on a magnetic substrate 53, and open regions 65 and 67 are opened by patterning the insulating film 59a. Next, as shown in FIG. 8B, a lead terminal portion 71 is formed in the vicinity of, for example, the open region 65 on the insulating film 59a. Next, as shown in FIG. 8C, an insulating film 59b is formed on the entire surface, and a contact hole 75 in which the lead terminal portion 71 is exposed and openings at the open regions 65 and 67 are formed by patterning the insulating film 59b. 
Next, as shown in FIG. 8D, a coil conductor 61 patterned into a spiral shape is formed on the insulating film 59b by using a frame plating method. One end of the coil conductor 61 is formed on the lead terminal portion 71 exposed in the contact hole 75. As a result, the coil conductor 61 and the lead terminal portion 71 are electrically connected to each other.
Next, as shown in FIG. 9A, an insulating film 59c is formed on the entire surface, and openings at the open regions 65 and 67 are formed by pattering the insulating film 59c. Next, as shown in FIG. 9B, a coil conductor 63 patterned into a spiral shape is formed on the insulating film 59c by using the frame plating method.
Next, as shown in FIG. 9C, an insulating film 59d is formed on the entire surface, and a contact hole 77 in which one end of the coil conductor 63 is exposed and openings at the open regions 65 and 67 are formed by patterning the insulating film 59d. 
Next, a lead terminal portion 73 is formed on the insulating film 59d. As shown in FIG. 9D, one end of the lead terminal portion 73 is formed on the one end of the coil conductor 63 exposed in the contact hole 77. As a result, the coil conductor 63 and the lead terminal portion 73 are electrically connected to each other.
Next, as shown in FIG. 10A, an insulating film 59e is formed on the entire surface, and the open regions 65 and 67 are opened by patterning insulating film 59e. Next, as shown in FIG. 10B, composite ferrite obtained by mixing magnetic powder of ferrite in insulating resin is embedded in the open regions 65 and 67 to form magnetic layers 68. Next, an adhesive is applied on the magnetic layers 68 in the open regions 65 and 67 and the insulating film 59e to form an adhesive layer 69. Next, a magnetic substrate 55 is attached to the adhesive layer 69, and the common mode choke coil is completed.
In order to improve impedance characteristics by increasing magnetic coupling between the coil conductors 61 and 63 and by increasing common impedance, the film thickness of the insulating film 59c between the coil conductors 61 and 63 is required to be made uniform and flat. Further, the film thicknesses of the coil conductors 61 and 63 are made uniform, and the sectional shapes of the coil conductors 61 and 63 orthogonal to the direction of the flow of current are required to be made almost identical independently of place.
However, each of the insulating films 59a to 59e is formed by applying, for example, polyimide resin on the entire surface and is patterned to provide the openings at the open regions 65 and 67. The polyimide resin applied in the open regions 65 and 67 is removed each time the respective insulating films 59a to 59e are formed, and the magnetic substrate 53 is always exposed in the open regions 65 and 67. Thus, a large step occurs between the upper surface of the insulating film and the surface of the magnetic substrate 53 exposed in the open regions 65 and 67. By the influence of the step, as shown in FIG. 9B, the film thickness of the insulating film 59c is decreased in the vicinity of the open regions 65 and 67, and the film thickness of the insulating film 59c in the vicinity of the open regions 65 and 67 is different from that in the other area, and as a result, there arises a problem that the interval between the coil conductors 61 and 63 becomes uneven.
Further, since the coil conductor 63 is formed on the insulating film 59c having the uneven film thickness, as shown in FIG. 9B, the sectional shape of the coil conductor 63 also becomes uneven between the vicinity of the open regions 65 and 67 and the other area. Also in the patterning of the resist frame for the formation of the coil conductor 63, the height of the resist frame becomes uneven between the vicinity of the open regions 65 and 67 and the other area, and there is a possibility that the plating film overflows onto the upper surface of the resist frame formed to have the low height, and the sectional shape of the coil conductor 63 becomes uneven. Further, there is also a problem that the resistance value of the coil conductor 63 locally varies due to the unevenness of the sectional shape of the coil conductor 63.
As stated above, when the film thickness of the insulating film 59c becomes uneven, the magnetic coupling between the coil conductors 61 and 63 becomes small, the common impedance is lowered, and the impedance characteristic is deteriorated. Further, the manufacture of the common mode choke coil having high impedance becomes difficult by the unevenness of the film thickness of the insulating film 59c and the unevenness of the sectional shape of the coil conductor 63.